Rotary retort furnaces are suited for heat treating relatively small parts, either by carburizing or neutral hardening techniques.
In this arrangement, the parts to be heat treated are fed into the interior of a rotating retort horizontally mounted in a furnace. The retort typically has helical flights formed along its inner walls. Rotation of the retort results in the movement of the parts progressively through the interior of the retort. The furnace is directly heated, and the parts are indirectly heated as they are conveyed through the rotating retort. During the progressive movement through the retort the parts are subjected to a desired heat treating atmosphere. The discharge end of the retort has an opening, where the parts fall through a discharge chute into an oil quench bath. The oil quench bath may have a conveyor to a move the parts to a washer and a tempering furnace in a continuous sequence.
Various furnace atmospheres are employed for heat treating. These are described in Metals Handbook, Ninth Edition, Volume 4, pages 393-399. The endothermic gas or gas atmosphere employed for heat treating is introduced at a pressure to provide a positive pressure in the retort to keep the room atmosphere out of the retort.
There is a predetermined temperature, or a predetermined range of temperatures, where the desired heat treating reaction occurs between the parts and the treatment atmosphere. Zone walls located within the furnace along the length of the retort create different temperature conditions within the retort to assure that, before the parts arrive at the discharge chute, their temperature has been properly elevated to the desired level. The furnace walls thereby create, by indirect heating, a discrete operative zone within the retort, located between its inlet and discharge ends, where the proper temperature conditions exist for the desired heat treatment process. On the inlet, side of this operative zone, the parts are indirectly preheated by the furnace. On the discharge side of this operative zone, the parts (now heat treated) lose heat as they enter the oil quench bath.
The discharge end of the retort is usually enclosed by a hood. The hood prevents the outside air from leaking into the discharge end of the retort. The hood is provided with an opening below the centerline of the retort to enable the parts to fall by gravity from the discharge end of the retort into the quench bath. The discharge chute is immersed in the quench bath to provide a liquid seal so that no room atmosphere will enter the retort through the discharge chute.
In retort, furnaces of this design, it is desirable to measure the heat treating atmosphere in the operative zone where the temperatures that are critical to the heat treatment process occur. However, prior attempts to accomplish this desireable objective have been unsuccessful. This is because the operative region of the retort is by necessity closed. Furthermore, the retort by necessity rotates within the furnace. Because of the structural configuration of rotary retort furnaces, there are few convenient places to insert a probe, except at its inlet or discharge ends.
In prior art attempts to monitor gas in rotating retorts, probes were mounted in a horizontal plane through the discharge end of the retort, where the parts fall into the quench bath. The horizontal mounting of the probe has been unsuccessful, because the probes did not have adequate structural integrity in a horizontal position to have a long useful life. Furthermore, these horizontal probes did not provide accurate results, because the oil vapors from the oil quench system adversely altered the probe measurements. In addition, since the treatment atmosphere is customarily introduced at the input end of the retort, by the time the atmosphere reaches the probe at the discharge end the retort, the sample can be influenced by contaminants introduced at the inlet of the retort, as well as introduced by the parts themselves as they are processed.
These probes did not provide accurate results for yet another reason. Being in the discharge end of the retort, the measurements of the probe were taken at temperatures different than the temperature critical for the treatment process, which occurs only in the operative zone.
Because of these problems, carbon control systems have not been commonly used in rotary retort furnaces. As a result, the heat treatment atmospheres have been inadequately monitored and controlled. Problems have occurred in the retort, including: sooting and plugging of the retort; contamination of parts and the quench bath; sticking of parts in the retort which causes mixing of parts of different sizes from one batch into another; clogging of carbon sensors; and reducing the life of the expensive alloy retort and hood casting components.